1. Field of the Invention
The present invention relates to a lift method and apparatus for moving an arm structure such as a boom of a crane.
2. Description of the Background Art
Various devices for moving an arm structure such as a lifting arm currently exist. These devices typically employ connecting plates joining relatively movable first and second arm structures. Known arrangements of this type require an excessive amount of force from a moving device that is attached to one of the arm structures and the connecting plate. Furthermore, the conventional art provides a limited range of movement of the second arm structure relative to the first arm.
For example, the conventional art of FIG. 9 shows a first arm structure 10 and a second arm structure 12 fastened together by connecting plate 14. Arm 12 pivots about pin 18, mounted in plate 14. A hydraulic piston and cylinder arrangement 16 is connected to the second arm structure 12 and to connecting plate 14. As shown with the dashed lines in FIG. 9, when the second arm structure 12 approaches a fully extended position, the movement of the second arm structure 12 with respect to the first arm structure 10 is limited by the orientation of the hydraulic piston and cylinder arrangement 16 relative to arm structure 12.
In the fully extended position, the hydraulic piston and cylinder arrangement 16 contacts the pin connection 18 that fastens the second arm structure 12 to the connecting plate 14. This limits the range of motion of arm 12.
As illustrated in FIG. 9, the conventional arm structure arrangement is highly inefficient when the second arm structure 12 approaches a fully extended position. This inefficiency can be explained with reference to the angle .theta. between longitudinal axis B1 of moving device 16 and longitudinal axis A1 of arm structure 12. This angle .theta..degree.0 affects the respective forces generated by the second arm structure 12 (denoted as F.sub.12) and the hydraulic piston and cylinder arrangement 16 (denoted as F.sub.16).
As the arm 12 approaches a fully extended position, a longitudinal axis B1 of the piston and cylinder 16 is nearly parallel to a longitudinal axis A1 of arm 12, where angle .theta. substantially approaches 0.degree.. The extending force F.sub.16 generated by the hydraulic piston and cylinder arrangement 16 is nearly perpendicular to force F.sub.12, which is the component of force tending to rotate arm 12 counter-clockwise due to the weight of arm 12 plus any loads carried by arm 12. Force F.sub.16, which has component forces F.sub.16a and F.sub.l6b, therefore, cannot efficiently counteract force F.sub.12.
Accordingly, a need in the art exists for a lift method and apparatus for moving an arm structure that substantially increases a range of motion of a second arm structure relative to a first arm structure and which also provides a moving device with a mechanical advantage whereby work required to move the second arm structure relative to the first arm structure is substantially reduced while lifting capacity of the moving device is substantial increased.